It is well known that in the combustion of carbonaceous fuels, such as coal, the sulphur contained therein is oxidised, usually largely to sulphur dioxide, although under certain conditions some sulphur trioxide can be formed. Whilst a small proportion of this sulphur dioxide can be chemically combined as sulphite (or sulphate if sulphur trioxide is present) into the ash formed largely from the non-combustible materials in the fuel, most of the sulphur dioxide is vented from the furnace combustion zone as part of the exhaust gasses. In the past these furnace exhaust gasses containing sulphur dioxide have been vented to the atmosphere through a stack, but this is no longer possible, due to ecological damage caused by such acidic emissions, including the formation of acid rain. The only known effective way to reduce the release of sulphur oxides to an acceptable level is to capture the sulphur oxides chemically from the furnace exhaust gasses before they are vented to atmosphere.
One method that is extensively used to capture sulphur oxides is to add an alkaline reacting solid additive to the hot furnace gasses to react with the sulphur oxides. The most commonly used additive is a particulate material derived from limestone, and it is usually added to the combustion zone of the furnace. Four reactions involving the limestone material are theoretically possible:
(a) CaCO.sub.3.fwdarw.CaO+CO.sub.2, PA1 (b) CaO+SO.sub.2.fwdarw.CaSO.sub.3, PA1 (c) 2CaO+2SO.sub.2 +O.sub.2.fwdarw.2CaSO.sub.4, and PA1 (d) CaO+SO.sub.3.fwdarw.CaSO.sub.4. PA1 (a) recovering calcium oxide containing ashes from the fluidised bed combustor; PA1 (b) mixing the recovered ashes with at least enough water to provide full hydration of the calcium oxide contained in the ashes; PA1 (c) transferring the mixture of ashes and water to a sonochemical reactor; PA1 (d) exposing the mixture of water and ashes in the reactor to sonic radiation at a power level and at a frequency sufficient to cause the hydration reaction to proceed to an acceptable level; and PA1 (e) recovering from the reactor a sufficiently hydrated ash product. PA1 (i) CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2 PA1 (ii ) CO.sub.2 +H.sub.2 O.fwdarw.H.sub.2 CO.sub.3 PA1 (iii) Ca(OH).sub.2 +2H.sub.2 CO.sub.3.fwdarw.Ca(CO.sub.3).sub.2 +2H.sub.2 O. PA1 (i) receiving a flow of calcium oxide containing ashes from the fluidised bed combustor; PA1 (ii) mixing the ashes with at least enough water to provide full hydration of the calcium oxide contained in the ashes; PA1 (iii) providing to the mixture of ashes and water sufficient carbon dioxide to convert a desired amount of the hydrated calcium oxide to calcium carbonate; PA1 (iv) transferring the mixture of ashes and water either before or after step (iii) to a sonochemical reactor; PA1 (v) exposing the mixture of water, carbon dioxide and ashes in the reactor to sonic radiation at a power level and at a frequency sufficient to cause the hydration reaction to proceed to an acceptable level; and PA1 (vi) recovering from the reactor a hydrated ash product containing calcium carbonate.
In the conditions that exist in the combustion zone of a fluidised bed combustor furnace, or FBC, sulphur trioxide is generally present in only relatively low levels, so that the last of these reactions is of little importance. As a result, the furnace ash residues typically comprise a heterogeneous mixture of CaO, CaSO.sub.4, limestone, unreacted carbonaceous char derived from the fuel and fuel derived ash materials, which are primarily inorganic compounds. This technique is particularly suitable for modern fluidised bed combustors (FBC's).
All of these known processes utilising a more or less dry particulate additive show poor utilisation of the added reagent, in the sense that in order to reduce significantly the amounts of sulphur dioxide, and of sulphur trioxide if present, in the furnace gasses prior to venting to the atmosphere, a substantial excess of the reagent has to be used, above the theoretical requirements of the reactions set out earlier. This is particularly true for the most commonly used reagent, which is either limestone as such (substantially CaCO.sub.3), lime (CaO), or hydrated lime (ca(OH).sub.2 plus some CaO). Although limestone is a relatively low cost material, both limestone, lime and hydrated lime are poorly utilised in the sulphur dioxide capture process, with utilisation figures in the range of 30%-40% being considered good.
Poor utilisation of the lime or limestone with its adverse effect on furnace operation costs, also provides a furnace ash which poses disposal problems. Since sulphur dioxide capture in the furnace is generally inefficient, FBC ashes commonly contain up to at least about 20% of free CaO. An FBC ash containing this amount free CaO cannot simply be dumped. It can generate dangerously high temperatures in contact with water, and landfill sites containing it are both unstable and generate a water leachate with an unacceptably high alkaline pH in the range of between 11 and 12. This leachate too requires treatment before it can be safely discharged. Further, over extended time periods in such a landfill site these ashes are found to be subject to considerable expansion, which both affects dump stability and produces yet more alkaline leachate requiring treatment.
In order to mitigate these difficulties, FBC ashes are generally subjected to a two stage CaO hydration procedure. First, the ash solids are mixed with water, generally in a pug mill. Then the wet solids are treated with further water at the disposal site, in part to complete the hydration process and in part to achieve optimum solids density. The second addition of water allows cementitious reactions involving the other components in the ash to go to completion, which should improve the overall strength and durability of the landfill site.
This method suffers from several disadvantages. Chemical analysis of the hydrated ash shows that at the end of the two stage process the hydration reaction is not complete, and at most only about 70-80% of the CaO in the ash is hydrated. It is also found that the water losses encountered due to steam formation in hydrating the ash are quite high, and can range as high as 40-50% by weight of the ash being treated, even though the theoretical water requirement for an average ash containing about 18% free CaO is only approximately 6% by weight of the ash being treated. It is also found that the hydration reaction at ambient temperatures is slow, and may take hours, or even days, to reach a reasonable level of completion.
Several methods have been proposed whereby better hydration of FBC ashes may be obtained.
It has been proposed to increase the reaction rate by increasing the water temperature. In the so-called Pyropower method, water at 98.degree. C. is recommended. In the so-called CERCHAR process a pressurised hydration reactor is used. Both of these methods whilst proffering a better level of hydration, increase significantly the cost of the hydration process.
A need therefore exists for a faster, less expensive and more effective way of hydrating at least a major portion of the CaO content of FBC ashes.